Field of the Invention
Embodiments of the invention relate to a touch sensing system and a method for driving the same.
Discussion of the Related Art
User interface (UI) is configured so that users are able to communicate with various electronic devices and thus can easily and comfortably control the electronic devices as they desire. Examples of the user interface include a keypad, a keyboard, a mouse, an on-screen display (OSD), and a remote controller having an infrared communication function or a radio frequency (RF) communication function. User interface technology has continuously expanded to increase user's sensibility and handling convenience. The user interface has been recently developed to include touch UI, voice recognition UI, 3D UI, etc.
A capacitive touch sensing system includes a capacitive touch screen which has durability and definition better than an existing resistive touch screen and is able to recognize a multi-touch input and a proximity touch input. Hence, the capacitive touch sensing system may be applied to various applications.
The size of mobile information terminals using the touch UI is becoming larger and larger. Thus, the touch UI is expected to be applied to large-sized display devices such as computer monitors in the future. As the size of the touch screen increases, the length of lines used in the touch screen lengthens. Hence, a resistance and a capacitance of the touch screen increase, and an RC delay of a driving signal applied to the touch screen increases. FIGS. 1 and 2 illustrate examples of the RC delay.
As the size of the touch screen increases, the RC delay varies depending on a position of the touch screen. Therefore, an amount of charges supplied to touch sensors of the touch screen varies, and a discharge delay of undesired remaining charges is caused in the touch sensors. Hence, a signal-to-noise ratio (often abbreviated SNR) of the signal read from the touch screen is not good. Because a length of a sensing period of the touch screen increases due to an increase in the size of the touch screen, a touch report rate is reduced. When the touch report rate is reduced, touch sensitivity is further reduced. Touch raw data obtained by sensing the touch sensors of the touch screen during the sensing period is analyzed to calculate coordinates of the touch raw data, and coordinate informations of the touch raw data are gathered. The touch report rate is a velocity or a frequency, at which the gathered coordinate informations are transmitted to an external host system. As the touch report rate increases, a latency between a touch input and a coordinate recognition is reduced. Therefore, the touch sensitivity a user feels increases.
FIG. 1 illustrates a portion of a mutual capacitive touch screen. In FIG. 1, Tx1 to Tx5 denote Tx lines to which a driving signal is applied, and Rx1 to Rx6 denote Rx lines receiving voltages of touch sensors Cm. A mutual capacitive touch screen TSP is connected to a readout integrated circuit (ROIC) which drives the touch screen TSP and receives the voltages of the touch sensors Cm. The ROIC applies the driving signal to the Tx lines Tx1 to Tx5 and receives the voltages of the touch sensors Cm through the Rx lines Rx1 to Rx6.
If the touch sensors Cm close to the ROIC have a small RC delay, there is an increase in an amount of charges ΔQ1 (refer to FIG. 2) charged when the driving signal is applied to the touch sensors Cm close to the ROIC. On the other hand, if the touch sensors Cm far away from the ROIC have a large RC delay, there is a reduction in an amount of charges ΔQ2 (refer to FIG. 2) charged when the driving signal is applied to the touch sensors Cm far away from the ROIC. As the RC delay increases, a discharge time of the touch sensors Cm increases. Thus, as the touch sensors Cm are far away from a portion of the touch screen TSP to which the driving signal is applied, an amount of charges charged to the touch sensors Cm decreases due to the RC delay, and a discharge of remaining charges is delayed.